Since ancient times, numerous surveying devices have been known for surveying a target point. In this context, the direction and angle and usually also the distance from a surveying device to the target point to be surveyed have been recorded as spatial standard data, for example by means of laser radiation, and, in particular, the absolute position of the surveying device together with possibly present reference points has been detected.
Generally known examples of such surveying devices consist of theodolites, tachymeters and total stations, also referred to as an electronic tachymeter or computer tachymeter. A geodetic surveying device according to the prior art is described, for example, in the publication EP 1,686,350. Such devices have an electrosensory angle surveying function and, under certain circumstances, distance surveying function, which functions permit the direction and distance from a selected target to be determined. For the determination of the direction and distance, the devices generally have a radiation source for emitting surveying radiation such as e.g. laser radiation.
The angle variables or distance variables are ascertained here in the interior reference system of the device and under certain circumstances also have to be linked to an external reference system in order to determine absolute positions.
In many geodetic applications, target points are surveyed by positioning specially configured target objects there or mounting them on a mobile vehicle, e.g. a construction machine, with the result of a geodetic surveying system composed of a surveying device and a target object is obtained. Such a target object as a surveying aid instrument is composed, for example, of a retro-reflector (e.g. a 360° prism) which is mounted e.g. on a plumb staff and which retro-reflects laser radiation emitted by the total station. However, surveying systems which operate without a reflector are also possible, such as are described, for example in the European patent application with the application number EP 10168771.3.
In order to sight the target point to be surveyed, geodetic surveying devices of the generic type have a telescopic sight, such as e.g. an optical telescope, as a sighting or targeting device. The telescopic sight is generally rotatable about a vertical axis and about a horizontal tilting axis relative to a base of the surveying device, with the result that the telescopic sight can be aligned with the point to be surveyed by pivoting and tilting. Modern devices can have, in addition to the optical viewing channel, a camera for capturing an image, said camera being integrated into the telescopic sight and aligned, for example, coaxially or in parallel, wherein the captured image can be represented, in particular, as a live image on the display of the display control unit and/or on a display of the peripheral device—such as e.g. the data logger—which is used for remote control. In this context, the optical system of the targeting device can have a manual focus—for example an adjusting screw for changing the position of a focusing optical system—or an autofocus, wherein the focus position is changed e.g. by servomotors. For example, such a targeting device of a geodetic surveying device is described in European patent application No. 09152540.2. Automatic focusing devices for telescopic sights of geodetic devices are known e.g. from DE 197 107 22 or DE 199 495 80. For example, the design of generic telescopic sights of geodetic devices is presented in the publications EP 1 081 459 and EP 1 662 278.
Modern total stations have microprocessors for digital further processing and storage of acquired measurement data. The systems generally have a compact and integrated design, wherein usually coaxial distance surveying elements and computational units, control units and storage units, for example in the form of a control and evaluation unit, are present in a device. Depending on the level of expansion of the total station, it is additionally possible for motorization of the sighting and targeting device and—in the case where retro-reflectors (for example of a 360° prism) are used as target objects—means for automatic target finding and tracking to be integrated. The total station can have an electronic display control unit as a human/machine interface—generally a microprocessor computer unit with electronic data storage means—with display and input means, e.g. a keyboard. The measurement data which is acquired by electrosensory means is fed to the display control unit, with the result that the position of the target point can be ascertained, displayed visually and stored by means of the display control unit. Total stations which are known from the prior art can also have a radio data interface for establishing a radio link to external peripheral components, such as e.g. to a portable data acquisition device which can be embodied, in particular, as a data logger or field computer.
Surveying devices with a further expansion level have a fine-sighting unit and an automated target fine-sighting function or target-tracking function for prisms which serve as target reflectors (ATR: “Automatic Target Recognition”). For this, a separate ATR light source, e.g. a further laser source, for emitting an ATR measurement beam or fine-sighting beam and a large-area ATR detector (e.g. CCD large-area sensor) which is sensitive to the emission wavelength of this light source are jointly additionally integrated into the telescope.
Within the scope of the ATR fine-sighting function and ATR target-tracking function the ATR measurement beam is emitted here in the direction of the optical target axis of the targeting device. Said ATR measurement beam is retro-reflected, for example, at a 360° prism (as the target reflector), and the reflected beam is sensed by the ATR sensor. Depending on the deviation of the alignment from the optical target axis of the 360° prism, the impinge position of the reflected radiation on the ATR sensor also differs from a central sensor surface position (i.e. the reflection spot of the ATR measurement beam which is retro-reflected at the prism on the ATR large-area sensor is, for example, not located in the center of the ATR large-area sensor and therefore does not impinge at a setpoint position which has been defined, e.g. on the basis of calibration, as that position which corresponds to the optical target axis).
If this is the case, the alignment of the sighting device is usually slightly readjusted in a motorized fashion in such a way that the ATR measurement beam which is retro-reflected at the prism impinges with high precision in the center of the sensor surface on the ATR large-area sensor (i.e. the horizontal and vertical angle of the targeting device are changed and adapted iteratively until the center of the reflection spot coincides with the setpoint position on the ATR large-area sensor). It is then often stated that the target is “locked onto”.
Besides the ATR fine-sighting function, an automatic target-tracking functionality can also be made available in a similar way by using the same ATR components (such as ATR light source and ATR detector). After ATR fine sighting has taken place (i.e. after the targeting device is aligned with the target in such a way that the center of the ATR measurement beam reflection spot coincides with the setpoint position—corresponding to the target axis—on the ATR large-area sensor), the targeting device can then continue to track movements of the target “live” and at corresponding speed in such a way the center of the ATR measurement beam reflection spot still remains as accurately as possible always at the setpoint position on the ATR large-area sensor.
In order to ensure the functioning of the automatic sighting on the basis of the evaluation of the position of the reflection spot of the ATR measurement beam, which is retro-reflected at the prism, at the ATR large-area sensor it is necessary, before the function starts, to align the sighting device with the target reflector at least approximately in such a way that the ATR measurement beam in general impinges on the prism and, having been reflected from there, on the ATR large-area sensor. In other words, the target must firstly be found so that the targeting device can be at least approximately aligned with the target. Such finding of the target is also necessary alongside the initial approximate alignment which takes place at the start of the measurement if the target moves suddenly and quickly in such a way that it disappears from the field of vision of the ATR detector (i.e. ATR measurement radiation reflected at the target no longer impinges on the ATR large-area sensor) and therefore the “lock-on” has been lost. Other causes of an interruption of the optical link between the total station and the target object may be, for example, unfavorable environmental conditions (precipitation, fog, dust etc.) or simply visual obstacles that block the optical link.
The approximate alignment of the sighting device with the target object so that the ATR measurement beam which is reflected by said target object impinges on the ATR sensor can occur e.g. by means of manual sighting of the target reflector by measurement by eye. However, the automated theodolites or total stations which are common nowadays are equipped with an optoelectronic target seeking and positioning device, referred to below as an automatic target detection unit or target-finding unit (AZE). Such theodolites are able to move automatically toward the target point and usually also at least to determine its position approximately. By means of the AZE it is therefore possible to align the sighting device or the target axis automatically at least approximately with the target object in such a way that fine sighting and/or target tracking is then possible by means of the ATR components. When the function operates satisfactorily, the saving in time with such automated instruments compared to approximate manual alignment is considerable.
A geodetic surveying system with an AZE according to the prior art is proposed, for example, in U.S. Pat. No. 6,035,254. According to this patent, the total station as the surveying device and the target object are respectively equipped with a receiver for receiving GPS data. Position information for estimating the position of the target object from received GPS data is transmitted to the total station and is used at the total station to determine how the total station has to be aligned for sighting of the target object. However, such satellite-supported AZE requires a free line of sight to a plurality of satellites. Use in narrow passages between houses or under trees or bridges or inside tunnels or buildings is therefore disadvantageously restricted or impossible. In addition, sighting of a moving target is disadvantageously not possible or at least not possible in a robust fashion.
European patent EP 1,329,690 discloses a method for automatically finding a geodetic target object, wherein radiation which is emitted in a fan which is aligned perpendicularly and which is pivoted horizontally is generated by a radiation transmitter unit of an AZE, e.g. by means of a pulse laser diode. If the AZE radiation impinges on the target, a portion of the radiation is reflected and received by a receiver unit, on the basis of which a horizontal angle with respect to the target is determined. It is disadvantageous that with the method according to EP 1,329,690, it is also not possible to sight a moving target, or at least not in a robust fashion.